Endoprosthesis with a ball-and-socket joint

ABSTRACT

In an endoprosthesis including a ball-and-socket joint, the ball and a socket insert having a spherical surface are made of ceramic material. The opening edge of the socket is covered by a polyethylene protecting ring whose opening side is covered by a covering ring made of a resilient material. The inner edge of the protecting ring is appropriately covered by the protecting. The socket, the ceramic insert, the protective ring and the covering ring are configured so as to permit sliding of a ball seated in the socket and configured to conform to the spherical surface of said ceramic insert out of the endoprosthesis.

It is known (EP-A 53794) to equip the socket of a ball-and-socketendoprosthesis, for example a hip prosthesis, with a slide surface whichis made of ceramic material. If, in the event of so-called subluxation,the ball guided therein moves partially away from the intended bearingposition, line contact with the edge of the slide surface may occur,which can lead to damage of both the ball and also the edge of thesocket. It has thus been proposed to place a protective ring made ofplastically deformable polyethylene in front of the edge of the slidesurface (EP-A 53794).

This protective ring is not normally subject to any wear. However, itmay happen that in the event of a strongly projecting movement, the neckcarrying the ball will strike the polyethylene ring. This can lead todamaging friction. It has also been found that the protective ring maybe deformed in the process, so that its function of protecting theceramic part is impaired.

According to the invention, these disadvantages are avoided by the factthat the front side of the protective ring is covered by a covering ringmade of resistant material. This material may be a metal or even aresistant plastic. To ensure that the surface of the ball cannot strikethe covering ring and become damaged in the event of subluxation, theprotective ring is expediently designed in such a way that it covers theinner edge of the covering ring.

The invention is explained in greater detail hereinbelow, with referencebeing made to the drawing which depicts an advantageous illustrativeembodiment and in which:

FIG. 1 shows an enlarged longitudinal section through a hip-jointendoprosthesis, and

FIG. 2 shows a still further enlarged partial section through the edgeof the socket.

The hip socket 1 comprises a holder 2, a polyethylene body 3 and aceramic insert 4. The holder 2 is externally designed such that it canbe anchored easily and permanently in the bone. For this purpose, it isprovided, for example, with projections 5. For details of this,reference may be made to the prior art.

It has a frustoconical inner shape 6, 7 to which the outer shape of thepolyethylene body 3 corresponds. After implantation of the holder 2,this outer shape can be fitted therein essentially free of play andanchored. This too is known.

The polyethylene body 3 has a frustoconical inner shape 8, 9 to whichthe outer shape of the ceramic insert 4 corresponds free of play, ifappropriate with a slight oversize. This ceramic insert 4 consists of aphysiologically compatible ceramic material which exhibits a high degreeof resistance to wear, for example high-purity aluminium oxide ceramic.It forms a spherical slide surface 10 which corresponds to the sphericalconfiguration of the ball 11 which is mounted therein. The sphericalslide surface 10 extends in each direction over approximately 180°; theequator of the spherical surface forming the slide surface 10 liessomewhere between the start of the edge rounding 12 and the front side13.

Applied to the front side 13 of the ceramic insert there is a protectivering 14 made of high-density polyethylene or similar resilient andpreferably sufficiently elastic material, which ring 14 is in turncovered over a large extent by a covering ring 15 made of suitablematerial, for example titanium. In the example depicted, this coveringring 15 is designed as a threaded ring which cooperates with thepolyethylene body 3 via a thread 16 in order to secure the ceramicinsert 4 and the protective ring 14 in position. A securing means (notshown) can be provided in order to prevent undesired loosening of thecovering ring 15.

The polyethylene body 3 transmits force in a uniformly distributedmanner from the ceramic body 4 to the holder 2 and in this way protectsagainst harmful stress concentrations on the ceramic body 4. Likewise,the protective ring 14 distributes the retaining force, originating fromthe covering ring 15, to the front side 13 of the ceramic body 4 whileavoiding stress peaks.

The protective ring 14 preferably situated on the other side of theequator--as viewed from the slide surface 10 of the ceramic body4--projects inwards almost to the imaginary spherical surface whichcontinues the slide surface 10 on the opening side. On the one hand, itexpediently has a little play with respect to the ball 11, when thelatter is lying properly on the slide surface 10, so that frictionalwear of the polyethylene is avoided and the joint fluid has free accessto the slide gap. On the other hand, it is expedient if its internaldiameter is a little smaller than the maximum diameter of the ball 11,in order to form a force barrier preventing the ball from sliding out ofthe socket. Most subluxations can be prevented in this way. However, theforce barrier should be smaller than the force at which the ball 11 candetach from the cone 17 bearing it.

The covering ring 15 protects the protective ring 14 from possiblydamaging impacts from the ball neck 18. Because of this protection, itcan be produced with greatly reduced dimensions. Its thickness is, forexample, of the order of one millimetre. On its inner edge it has acollar 19 which extends axially parallel outwards and which lies infront of the inner edge 20 of the covering ring 15 and so protectsagainst the ball 11 coming into potentially damaging direct contact withthe covering ring 15 which is made of harder material. As can be seen inFIG. 2, the collar 19 can even protrude slightly over the edge of thecovering ring 15 on the opening side. This admittedly means that theneck 18 can strike against the collar 19, but this does not cause anydamage as long as the latter can deflect elastically. Its elasticdeflection is possible by virtue of the fact that a wedge-shaped gapopens between the opening-side edge rounding 12 of the ceramic insertand the protective ring 14, into which wedge-shaped gap the inner edgeof the protective ring can deflect back elastically in such a

What is claimed is:
 1. An endoprosthesis, comprising:a socket comprisinga ceramic insert having a spherical slide surface and forming an openingedge; a protective ring made of a resilient material and covering saidopening edge of said ceramic insert; and a covering ring provided on afront side of said protective ring and covering said protectivering,said socket, said ceramic insert, said protective ring and saidcovering ring being configured upon implantation and assembly so as topermit sliding out of said endoprosthesis of a ball seated in saidsocket just by pulling the ball from the socket, said ball beingconfigured to conform to said spherical surface of said ceramic insert.2. The endoprosthesis of claim 1, wherein said protective ring comprisespolyethylene.
 3. The endoprosthesis of claim 1, further comprising acollar between said protective ring and said ceramic insert.
 4. Theendoprosthesis of claim 3, wherein said collar extends axially outward.5. The endoprosthesis of claim 3, wherein said collar protrudes over thecovering ring.
 6. The endoprosthesis of claim 1, wherein said coveringring comprises titanium.
 7. An endoprosthesis, comprising:a socketcomprising a polyethylene body and a ceramic insert, said ceramic inserthaving a spherical slide surface and forming an opening edge; aprotective ring made of a resilient material and covering said openingedge of said ceramic insert; and a covering ring provided on a frontside of said protective ring and covering said protective ring andcooperating with said polyethylene body,said socket, said ceramicinsert, said protective ring and said covering ring being configuredupon implantation and assembly so as to permit sliding out of saidendoprosthesis of a ball seated in said socket just by pulling the ballfrom the socket, said ball being configured to conform to said sphericalsurface of said ceramic insert.
 8. The endoprosthesis of claim 7,wherein said protective ring comprises polyethylene.
 9. Theendoprosthesis of claim 7, further comprising a collar between saidprotective ring and said ceramic insert.
 10. The endoprosthesis of claim9, wherein said collar extends axially outward.
 11. The endoprosthesisof claim 9, wherein said collar protrudes over the covering ring. 12.The endoprosthesis of claim 7, wherein said covering ring comprisestitanium.
 13. An endoprosthesis comprising a ball and a socket having aslide surface formed by a ceramic insert in said socket, said ceramicinsert comprising an opening edge; a protective ring in contact withsaid opening edge; and a covering ring covering a front side of saidprotective ring, said covering ring comprising an inner edge,saidsocket, said ceramic insert, said protective ring and said covering ringbeing configured upon implantation and assembly so as to permit slidingof said ball seated in said socket out of said socket just by pullingout the ball from said socket.
 14. The endoprosthesis of claim 13,wherein said opening edge of said ceramic insert is rounded.
 15. Theendoprosthesis of claim 14, wherein said rounded edge forms awedge-shaped region between said ceramic insert and said protectivering.
 16. The endoprosthesis of claim 5, further comprising a ballmounted in said socket and having an outer surface configured to conformto said spherical slide surface.
 17. The endoprosthesis of claim 7,wherein said covering ring engages with said polyethylene body viathreads.